The present invention relates generally to active matrix displays, and more particularly to active matrix displays having light emitting diodes.
FIG. 1A shows a section of an active matrix display with pixel elements including light emitting diodes. The section of an active matrix display in FIG. 1A includes a matrix of pixel elements, an array of selection driving lines, and an array of data driving lines that is essentially perpendicular to the array of selection driving lines. A pixel element 100 generally is connected to a selection driving line 300 and a data driving line 200.
As shown in FIG. 1B, a pixel element 100 includes a light emitting diode 110, a biasing transistor 50, a switching transistor 40, and a capacitor 48. Biasing transistor 50 has a gate 55, a first terminal 52, and a second terminal 54. Gate 55 is connected to capacitor 48, first terminal 52 is connected to light emitting diode 110, and second terminal 54 is connected to a common voltage Vz. Capacitor 48 is also connected to common voltage Vz. In addition to connecting to first terminal 52, light emitting diode 110 is connected a common voltage Vc. Switching transistor 40 has a gate 45, a first terminal 42, and a second terminal 44. Gate 45 is connected to a selection driving line 300, first terminal 42 is connected to a data driving line 200, and second terminal 44 is connected to gate 55 of biasing transistor 50.
During operation, pixel element 100 generally can be either in a data-setting mode or in a light-emitting mode. When pixel element 100 is in the data-setting mode, a selection signal (e.g., a selection voltage) on selection driving line 300 drives switching transistor 40 into a conducting state. When switching transistor 40 is in the conducting state, a semiconductor channel between first terminal 42 and second terminal 44 is essentially conductive, and a data signal (e.g., a data voltage) on data driving line 200 can set a gate voltage at gate 55 of biasing transistor 50 to a target voltage value. When pixel element 100 is in the light-emitting mode, a deselect signal (e.g., a deselect voltage) on selection driving line 300 drives switching transistor 40 into a non-conducting state. When switching transistor 40 is in the non-conducting state, a semiconductor channel between first terminal 42 and second terminal 44 is essentially non-conductive, and a gate voltage at gate 55 of biasing transistor 50 can be substantially maintained.
In general, a driving current I0 passing through light emitting diode 110 is determined by the gate voltage at gate 55 of biasing transistor 50. But, the driving current I0 passing through light emitting diode 110 also depends on some properties of biasing transistor 50. The biasing transistor 50 in different pixel element 100 may have different properties. Therefore, in certain applications, it is desirable to provide a pixel element 100 that includes a sensing output that may provide some measurement on the driving current I0 when the sensing output is enabled.